5 ms·

Wait, what is wrong with a digital thermostat?

by fumar 5y ago

sojournerc 5y ago
There can be such thing as too much precision. I have in-floor radiant heat that benefits from a longer duty cycle. I buy thermostats that are designed not just for a particular temp, but will also run without what I would call thrashing. E.g Constantly on and off because someone walked by and caused a breeze of slightly cooler air. Not to say a digital thermostat couldn't account for this (you have to find them specifically), and are probably better with good firmware, but murcury will have a wider margin before activation.
- seszett 5y ago
  What problem does adding hysteresis solve on a digital thermostat, though? Is it a problem that it stays closer to the asked temperature?
  - mjevans 5y ago
    If you've ever had a basic electronics course with a raw button, hysteresis might instead be known as 'debounce' or making the button's single human action a single 'press' result on the other side. I think a digital thermostat could work well enough if they weren't made with a focus on being stupidly cheap or possibly somehow still absolutely energy efficient. * Wake up and respond FAST to user inputs. * Show the numbers it's thinking of big (running average over 5, 10, 15 min?), but also the current reading small (but readable) updated whenever it polls. * If there's a hold or timeout involved display that too, expose the user to the thought process, don't just be a nearly opaque box. * Be responsive in other ways. If I switch modes at the very least kill the old mode's 'calling' line right then, even if the new one gets a cooldown to prevent equipment issues.
    - seszett 5y ago
      But none of this seems to need hysteresis for the actual activation/deactivation of heating depending on temperature readings? One reason I could see is to preserve the life of electromagnetic relays, but modern thermostats should use solid state relays. These points also seem to be solved for any somewhat modern (less than 15 years old?) run of the mill thermostat. Generally they always show the current reading and give the temperature setting only when you ask for it though, that's the only difference I see with what you want.
      - mjevans 5y ago
        The comment was something like 'walking by' disrupting the air at the thermostat and that alone being enough to turn the furnace on at that exact moment. A running average, but also making it clear to the user (both the value the unit is using for the decision and the current instant input) as well as a 'running average' over a given window address that specific comment. I added my own frustrations on top since they're also part of the larger 'dumb down the UI' and make the device frustrating.
- marcosdumay 5y ago
  > but murcury will have a wider margin before activation. Well, digital thermostats will have as much of margin as you program them to. Until your comment, I assumed every digital thermostat had hysteresis since the 90's. I can't imagine why they wouldn't. The more expensive ones have PWM control of the heater or heat exchanger, but that's something extra beyond what the mechanical ones did (metal strip thermostats used to be designed with hysteresis too, what isn't something they automatically have).
  - willis936 5y ago
    It's a bang-bang controller. It's so simple that it's what I came up with for controlling power producers / sinks to fill / drain buffers (load matching) in modded minecraft many years ago. It's the very simplest way to do this and is far from optimal. The only thing it optimizes is simplicity of implementation. A PI loop tuned to either minimize error or transitions is going to perform better for whatever goal or balance of goals a user wants.
    - marcosdumay 5y ago
      A bang-bang controller can have hysteresis or not, it's as simple as turning off in a higher temperature than the one it turns on and putting a low-pass filter (a capacitor if you do it in hardware) after the temperature sensor. The simplest possible implementation of an electronic thermostat would be able to switch thousands of times per second, and this is completely useless for controlling anything mechanical. When you are making real controllers, you always need some extra complexity, but you don't need a lot of it to make a good one.
      - willis936 5y ago
        Filters add more latency than PI(D) loops. They're not used in practice because they perform worse. Would you trust a 20% tolerance capacitor when setting your poles?
- alexanderdmitri 5y ago
  Search for a digital thermostat with hysteresis options to solve this problem.
  - amluto 5y ago
    Or a high quality one designed for in-floor radiant heat, e.g. Tekmar 552. Very simple to set up, very few gimmicks, and pretty much just works. (The Tekmar smart home interface for it is offensively priced. The thermostat works fine without it, though.)
- leetcrew 5y ago
  if true, this is kinda sad. I use a free utility that does this with my pc fans and lets me choose exactly what the margin is. how hard would this have been to add to nest?
dylan604 5y ago
The same thing as all digital to analog device comparisons. There is so much more forgiveness in analog. I really feel for those that have never worked with analog and only know the coldness of digital 0s and 1s. Digital is like living in a world of only black & white where analog ha[d|s] shades of gray. Take an Arudino with both analog and digital pins. A digital pin is either high (1) or low (0). That's it. On or Off. An analog pin can have a range of values from 0v to 3.3v or 5v depending on configuration. That single pin can be wired to several different items that each have a separate resistance allowing them to be identified by their passed voltage. At the same time, analog has its own problems with that lack of precision, so everything needs to accept a range. Put a resistance on a 5v line, and read from it multiple times and you get multiple readings, but within a +/- tolerance.

Wait, what is wrong with a digital thermostat?

by fumar 5y ago

Wait, what is wrong with a digital thermostat?

There can be such thing as too much precision. I have in-floor radiant heat that benefits from a longer duty cycle. I buy thermostats that are designed not just for a particular temp, but will also run without what I would call thrashing. E.g Constantly on and off because someone walked by and caused a breeze of slightly cooler air. Not to say a digital thermostat couldn't account for this (you have to find them specifically), and are probably better with good firmware, but murcury will have a wider margin before activation.

seszett 5y ago
What problem does adding hysteresis solve on a digital thermostat, though? Is it a problem that it stays closer to the asked temperature?
- mjevans 5y ago
  If you've ever had a basic electronics course with a raw button, hysteresis might instead be known as 'debounce' or making the button's single human action a single 'press' result on the other side. I think a digital thermostat could work well enough if they weren't made with a focus on being stupidly cheap or possibly somehow still absolutely energy efficient. * Wake up and respond FAST to user inputs. * Show the numbers it's thinking of big (running average over 5, 10, 15 min?), but also the current reading small (but readable) updated whenever it polls. * If there's a hold or timeout involved display that too, expose the user to the thought process, don't just be a nearly opaque box. * Be responsive in other ways. If I switch modes at the very least kill the old mode's 'calling' line right then, even if the new one gets a cooldown to prevent equipment issues.
  - seszett 5y ago
    But none of this seems to need hysteresis for the actual activation/deactivation of heating depending on temperature readings? One reason I could see is to preserve the life of electromagnetic relays, but modern thermostats should use solid state relays. These points also seem to be solved for any somewhat modern (less than 15 years old?) run of the mill thermostat. Generally they always show the current reading and give the temperature setting only when you ask for it though, that's the only difference I see with what you want.
    - mjevans 5y ago
      The comment was something like 'walking by' disrupting the air at the thermostat and that alone being enough to turn the furnace on at that exact moment. A running average, but also making it clear to the user (both the value the unit is using for the decision and the current instant input) as well as a 'running average' over a given window address that specific comment. I added my own frustrations on top since they're also part of the larger 'dumb down the UI' and make the device frustrating.
marcosdumay 5y ago
> but murcury will have a wider margin before activation. Well, digital thermostats will have as much of margin as you program them to. Until your comment, I assumed every digital thermostat had hysteresis since the 90's. I can't imagine why they wouldn't. The more expensive ones have PWM control of the heater or heat exchanger, but that's something extra beyond what the mechanical ones did (metal strip thermostats used to be designed with hysteresis too, what isn't something they automatically have).
- willis936 5y ago
  It's a bang-bang controller. It's so simple that it's what I came up with for controlling power producers / sinks to fill / drain buffers (load matching) in modded minecraft many years ago. It's the very simplest way to do this and is far from optimal. The only thing it optimizes is simplicity of implementation. A PI loop tuned to either minimize error or transitions is going to perform better for whatever goal or balance of goals a user wants.
  - marcosdumay 5y ago
    A bang-bang controller can have hysteresis or not, it's as simple as turning off in a higher temperature than the one it turns on and putting a low-pass filter (a capacitor if you do it in hardware) after the temperature sensor. The simplest possible implementation of an electronic thermostat would be able to switch thousands of times per second, and this is completely useless for controlling anything mechanical. When you are making real controllers, you always need some extra complexity, but you don't need a lot of it to make a good one.
    - willis936 5y ago
      Filters add more latency than PI(D) loops. They're not used in practice because they perform worse. Would you trust a 20% tolerance capacitor when setting your poles?
alexanderdmitri 5y ago
Search for a digital thermostat with hysteresis options to solve this problem.
- amluto 5y ago
  Or a high quality one designed for in-floor radiant heat, e.g. Tekmar 552. Very simple to set up, very few gimmicks, and pretty much just works. (The Tekmar smart home interface for it is offensively priced. The thermostat works fine without it, though.)
leetcrew 5y ago
if true, this is kinda sad. I use a free utility that does this with my pc fans and lets me choose exactly what the margin is. how hard would this have been to add to nest?

dylan604 5y ago

The same thing as all digital to analog device comparisons. There is so much more forgiveness in analog. I really feel for those that have never worked with analog and only know the coldness of digital 0s and 1s. Digital is like living in a world of only black & white where analog ha[d|s] shades of gray. Take an Arudino with both analog and digital pins. A digital pin is either high (1) or low (0). That's it. On or Off. An analog pin can have a range of values from 0v to 3.3v or 5v depending on configuration. That single pin can be wired to several different items that each have a separate resistance allowing them to be identified by their passed voltage. At the same time, analog has its own problems with that lack of precision, so everything needs to accept a range. Put a resistance on a 5v line, and read from it multiple times and you get multiple readings, but within a +/- tolerance.